1. Field of the Invention
The present invention relates to a high pressure fuel injection system for an internal combustion engine which includes a common rail for accumulating pressurized fuel and injectors for supplying fuel into cylinders of an internal combustion engine, and more particularly to a system in which a high pressure regulator for controlling pressure in the common rail is provided.
2. Description of Related Art
A fuel injection system in which a common rail for accumulating pressurized fuel therein is provided and fuel is injected into engine cylinders through injectors connected to the common rail has been known hitherto. For example, Japanese Utility Model Laid-Open Publication No. Hei-5-1854 and Japanese Patent Laid-Open Publication No. Hei-7-158536 disclose such a system which further includes a relief valve for relieving accumulated fuel pressure in the common rail when the fuel pressure therein exceeds a predetermined value. In other words, excessive fuel pressure is relieved to maintain it under the predetermined value.
It is also possible, in the known systems, to increase the fuel pressure in the common rail by operation of a fuel supply pump. It has not been possible, however, to control the fuel pressure during normal driving in such a way that the fuel pressure is quickly decreased in response to a sudden decrease of engine load, e.g., when an automatic transmission is shifted up from the second to the third gear, or when a driver releases an acceleration pedal. Generally, fuel quantity injected into an engine is controlled by changing an injection pulse width, i.e., the pulse width is decreased when engine load is lowered and increased when engine load becomes high. There is, however, a certain limitation to shorten the pulse width due to a response characteristic of the injector. Therefore, it is desirable to quickly decrease the fuel pressure in the common rail when smaller fuel quantity is required under certain conditions.
It is also required in this kind of fuel injection systems that vapor stored in the fuel system be discharged quickly when the engine is re-started under high ambient temperature, and that the system have a so called "limp home" ability, i.e., an ability at least to drive back home when the fuel system is in trouble. These requirements have not been properly fulfilled in the fuel systems known hitherto.